For example, FIG. 6 shows an audio component system which comprises audio components, i.e., a tuner 20, CD player 30, tape recorder 40 and MD player 50, which are connected to an amplifier unit 9. An audio signal sent out from one audio output component selected by the amplifier unit 9, e.g., from the CD player 30, is amplified and delivered as sound from a speaker 6 connected to the output terminal of the amplifier unit 9. Further one audio output component selected by the amplifier unit 9, e.g., the tuner 20, feeds an audio signal to one audio input component, e.g., to the MD player 50, which records the audio signal. The audio components 20, 30, 40, 50 have respective control circuits 21, 31, 41, 51 each adapted to control the operation of the component.
The amplifier unit 9 has a plurality of signal input terminals a, b, c, d having connected thereto signal output lines 81 to 84 extending from the audio output components 20, 30, 40, 50, and a plurality of signal output terminals e, f having connected thereto signal input lines 85, 86 extending from the audio input components 40, 50.
The signal input terminals a, b, c, d are connected via a selector 91 to a signal processing system 4, comprising a preamplifier 92, audio signal processing circuit 93 and post amplifier 94, and an audio signal is delivered to the speaker 6 via the signal processing system. The audio signal processing circuit 93 is adapted to process signals for volume adjustment and tone adjustment as specified by the user's manipulation. The output terminal of the preamplifier 92 is connected to the signal output terminals e, f through a plurality of on-off switches 95, 96, and an audio signal is delivered from the terminal e or f to the tape recorder 40 or MD player 50.
The selector 91 and the on-off switches 95, 96 are controlled by a control circuit 98 to realize an operation in accordance with the manipulation of an input unit 97 by the user. For example, when the audio signal to be produced by the CD player 30 is to be recorded on an MD (mini disk) by the MD player 50, the input unit 97 is manipulated to select the CD player 30 as a signal output component and the MD player 50 as a signal input component, whereupon the selector 91 is switched to the input terminal connected to the CD player 30, and the on-off switch 96 connected to the MD player 50 is closed.
Consequently, the audio signal from the CD player 30 is fed to the MD layer 50 by way of the selector 91, preamplifier 92 and on-off switch 96 and recorded on the MD.
However, the control circuit 98 of the amplifier unit 9 is unable to recognize whether all the components are connected to the amplifier unit 9 and whether all the components are energized to hold the control circuits thereof active, so that the conventional audio component system described has the problem that if the user manipulates the input unit 14 to increase the volume of the output sound, with the selector 91 selecting a signal input terminal actually connected to no component, a loud noise will be produced by the speaker 6 to cause discomfort.